1. Field of the Invention
The present invention relates to an improved device for measuring diametral changes in a cylindrical member and, more particularly, to such a device which measures diametral changes in a cylindrical member whereby axial loads on the cylindrical member may be determined.
2. Description of the Related Art
In many industries, it is important to measure the variable dynamic axial loads imposed on a work piece such as a cylindrical member. This is particularly true in the nuclear power industry where Motor Operator Valves (MOV's) are extensively used. It is important that these valves are set and maintained correctly at all times since correct operation is critical to health and safety as well as to proper operation of the system in which the MOV's are used. MOV's typically include a valve, a motor operator attached to the valve through a stem and yoke means extending partially around the valve stem for connecting the operator housing and the valve housing. The best measure for accurately monitoring the dynamic events within an MOV is by the direct measurement of the valve stem axial load.
It is possible to determine the axial load or strain in a valve stem or any other generally cylindrical member from changes in the stem or cylindrical member diameter. The ratio of diametral change to axial elongation for a material, referred to as Poisson's ratio, is known. Therefore, by measuring the diametral change on the valve stem or other cylindrical member, axial strain and therefore valve stem axial load can be determined.
There has long been need for a device which can accurately monitor and measure the dynamic operation of valves or other thrust bearing cylindrical members continuously during operation thereof.
One current device for determining the axial load on a valve stem is a device which senses the changes in clamp means attached around the valve stem. As diametral changes occur in the valve stem, the clamp portion moves in response to diametral changes in the clamped portion of the stem. A sensor is provided to sense movement of the clamp portion of the device in response to the diametral changes. The sensed changes in the clamp means may be bending in the clamp portion of the device or distance changes between parts of the clamp portion of the device. A signal storage device, which also may be a computer, stores the signals from the sensor for real time or delayed determination of axial loading in the stem or other cylindrical member. Such a device is shown, for example, in Leon et al. U.S. Pat. No. 4,911,004.
One disadvantage of the clamp type sensor such as that shown in Leon et al. U.S. Pat. No. 4,911,004 is that the clamp is quite bulky and cannot continuously travel with the stem during substantially the length of travel thereof. Moreover, the clamp can only be used to initially measure the stem load and this measurement is then used to calibrate another measuring device mounted on the yoke of the valve whereby the stem load is indirectly measured based on the strain in the valve yoke. This leads to many inaccuracies such as those caused by bending moments in the yoke, the fact that the yoke response is non-linear and can vary over the valve stroke and that the yoke load measuring device is sensitive to flow induced yoke vibration and valve stem harmonic imbalance. Additional inaccuracies in using a clamp type sensor can occur as a result of the fact that a stem has a normal stiffness and when the clamp is securely attached thereto, the normal stiffness changes.
A further problem with a method for determining valve stem load by measuring yoke strain is that the yoke sensor must be calibrated to a direct stem sensor prior to each use. Moreover, the yoke sensor must be permanently installed with one sensor per valve. Such sensors also can be difficult and time consuming to install.
Another device for measuring valve stem load is shown in Charbonneau et al. U.S. Pat. No. 4,542,649 which employs a system that measures displacement of a spring pack associated with the motor actuator of a MOV as an indicator of forces in the valve system. This spring pack deflection type device suffers from the disadvantage of being time consuming to install and calibrate. Moreover, spring pack deflection is proportional to motor torque and does not measure stem thrust. In addition, the spring pack deflection type devices can introduce errors of plus or minus 50% due to factors such as spring pack non-linearity, spring pack rate of loading and mechanical loss issues, the assumption that the spring pack deflection per unit thrust is the same in both directions and the fact that a spring pack device is calibrated in the out direction only into a load cell having a different stiffness than the valve seat.
Another device for measuring valve stem thrust loads in a cylindrical member such as a valve stem is disclosed in Branam et al. U.S. Pat. No. 4,856,327 wherein valve stem thrust loads in an MOV are monitored and measured by the use of load cells installed directly between the valve and the operator of the MOV. Again, however, this type of device does not take measurements directly on the valve stem and inaccuracies can occur as a result of vibration, temperature changes, etc.
Accordingly, there is still a need for an improved device for accurately and continuously directly monitoring and measuring the axial load in a cylindrical member such as a valve stem throughout a substantial portion of the length of travel of the cylindrical member and which as well may rotate with the cylindrical member. There is a further need for such a device which also can compensate for temperature changes in the cylindrical member as well as in the measuring device itself.